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Infrared spec (Aromatic signals (Weak C–H stretch at 3030 cm−1, Weak…
Infrared spec
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1600, 1500 and 1450 cm-1 are common
1660-2000cm(weak)
1450-1600 (medium)
absorb from around 1900 cm-1 to 1650 cm-1. Carbonyl absorptions are typically the strongest absorptions in IR spectra
Difficult to distinguish CO containing compounds based on ν(C=O) alone and need to use other information
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- Absorptions in the Fingerprint Region
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C-O bonds in alcohols, acids, ethers and esters strongly absorb around 1300-1100 cm-1
Otherwise there are absorption which may assist in determining the substitution pattern of aromatics and whether alkenes are cis, trans or gem.
- Hydrogen stretching region
Draw a line straight up from 3000 cm-1. Intensity on left is Csp2-H, to the right is Csp3-H
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There must be a change in the dipole moment of the molecule as a result of a molecular vibration (or rotation).
Symmetric molecules (or bonds) do not absorb IR radiation since there is no dipole moment. Double and triple bond inactive in IR
Practical Considerations
Can be done on solids, liquids (or gases!!)
a) Sodium chloride plates a) For liquids or Nugol ‘mulls’. Can be also used to identify different crystal polymorphs
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Infrared radiation is absorbed and converted by organic molecules into energy of molecular vibration
The higher the wavenumber, the higher the energy
The frequency or wavelength of the absorption depends on the relative massesof the atoms, the force constants of the bonds and the geometry of the atoms
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